The micronutrient B12 (cobalamin, Cbl) plays an essential role in homocysteine metabolism in most if not all cells in the body. Hyperhomocysteinemia is a major independent risk factor for cardiovascular disease and is also associated with other devastating conditions such as neural tube defects and Alzheimer's disease. The determinants of hyperhomocysteinemia are multifactorial and include both genetic and acquired conditions. Cobalamin transport, processing and coenzyme formation by cardiovascular cells and tissues is a neglected area of research. The long-term objectives of this proposal are to gain an understanding of 1) Cbl transport processes in cultured human aortic endothelial cells and smooth muscle cells, 2) Cbl processing within these cells, and 3) the conversion of Cbl vitamers to methylcobalamin and adenosylcobalamin, coenzymes for cytosolic methionine synthase and mitochondrial methylmalonyI-CoA mutase, respectively. This study is driven by the hypotheses that 1) the vascular endotheliurn plays an essential role in maintaining Cbl homeostasis throughout the body; 2) it does so by a//owing the Cbl-binding protein transcobalarnin to transcytose from the/urinal/(apical/) space to the ablurninal (basolateral) space with Cbl as its cargo; 3) endothelial ceil injury, as occurs in atherogenesis and atherosclerosis, may cause localized disruption of Cbl homeostasis; and 4) Cbl deficiency itself/f is a cause of vascular ceil dysfunction due to a breakdown of the hornocysteine rernethylation machinery, which leads to hyperhornocysteinernia. The Specific Aims of this proposal are: 1) to establish the mechanisms of Cbl transport and transcytosis in cultured human aortic endothelial and smooth muscle cells; 2) to determine how Cbl is processed in cultured vascular cells and the role that glutathionyl-Cbl plays in this processing; and 3) to elucidate the mechanisms of Cbl coenzyme formation in cultured human aortic endothelial and smooth muscle cells.